What’s in 1.7 stable

This release is primarily about library features. While we have several
language features cooking for future releases, the timeframe in which 1.7 was
developed included the holidays, which means less time for commenting on GitHub
and more time for spending with loved ones.

Library stabilizations

About 40 library functions and methods are now stable in 1.7. One of the
largest APIs stabilized was support for custom hash algorithms in the standard
library’s HashMap<K, V> type. Previously all hash maps would use SipHash as
the hashing algorithm, which provides protection against DOS attacks by
default. SipHash, however, is not very fast at hashing small keys. As shown,
however, the FNV hash algorithm is much faster for these size of inputs. This
means that by switching hash algorithms for types like HashMap<usize, V>
there can be a significant speedup so long as the loss of DOS protection is
acceptable.

To see this in action, you can check out the fnv crate on crates.io and
create a HashMap via:

Note that most of the time you don’t even need to specify the hasher as type
inference will take care of it, so HashMap::default() should be all you need
to get up to 2x faster hashes. It’s also worth pointing out that Hash trait
is agnostic to the hashing algorithm used, so no changes are needed to the
types being inserted into hash maps to reap the benefits!

Other notable improvements include:

<[T]>::clone_from_slice(), an efficient way to copy the data from one slice
and put it into another slice.

Various convenience methods on Ipv4Addr and Ipv6Addr, such as is_loopback(),
which returns true or false if the address is a loopback address according to
RFC 6890.

Various improvements to CString, used for FFI.

checked, saturated, and overflowing operations for various numeric types.
These aren’t counted in that ‘40’ number above, because there are a lot of
them, but they all do the same thing.

Cargo features

There were a few small updates to Cargo:

An improvement to build scripts that allows them to precisely inform Cargo
about dependencies to ensure that they’re only rerun when those files change.
This should help development quite a bit in repositories with build scripts.